Zekrom is best
It's got a badass design and pretty good typing (though as far as dual dragons go, there are much better ones out there) Plus, when you're fighting N, it feels kind of symbolic that you wield the sinister-looking Zekrom while N, friend to all Pokemon, wields the angelic Reshiram.
Of course, Zekrom isn't the only legendary I love. Following closely are Palkia (who is an example of a better typing, but its design is nowhere near as badass) Lugia (Movie 2000 was cooolllll, man) Deoxys (dat mecha alien) and Shaymin Sky Forme (I wish I could say I like Land Forme too, but not really :T) Basically, I'm a sucker for any mechanical monstrousity...and a flying hedgehog. Mewtwo also gets props for being an emotionally unstale genetic badass.